The quadrupole mass spectrometer, which uses a quadrupole mass filter as the mass analyzer, is one of the most widely used types of mass spectrometers because it is small in size and comparatively inexpensive. In quadrupole mass spectrometers, a direct-current voltage and a radio-frequency voltage are applied to each of the four rod electrodes constituting a quadrupole mass filter to create a quadrupole electric field within the space surrounded by the rod electrodes. This electric field oscillates ions while they are travelling through it, causing the ions which do not conform to specific conditions to be dispersed and removed halfway.
In general, a sample to be analyzed contains various foreign substances other than the target components. To perform qualitative and/or qualitative analysis of target components without being influenced by such foreign substances, chromatograph mass spectrometers, in which a mass spectrometer as previously described is combined with a chromatograph, such as a liquid chromatograph or gas chromatograph, are used. When a chromatograph mass spectrometer is used for a quantitative analysis of known target components, since the mass-to-charge ratio of each target component to be observed is known, the mass spectrometer is operated in a selected ion monitoring (SIM) measurement mode for repeatedly observing the intensity of the ions, focusing on one or a plurality of specific mass-to-charge ratios of interest. Using the data obtained by such a measurement, a data processing system creates an extracted ion chromatogram (which may also be called a mass chromatogram) which shows the relationship between the elapsed time and the ion intensity at the specific mass-to-charge ratio or ratios, then calculates the area of a peak appearing on that chromatogram at or near the retention time of a target component, and converts the area into the content of the target component (for example, see Patent Document 1). To enhance the quality of quantitative determination of target components by a chromatograph mass spectrometer, it is essential to improve the quality of the extracted ion chromatogram, or specifically, the S/N ratio of the chromatogram.
In a quadrupole mass spectrometer, the mass-resolving power can be regulated by changing the ratio between the radio-frequency voltage and the direct-current voltage applied to the rod electrodes of the quadrupole mass filter. Conventional chromatograph mass spectrometers also have such a function and allow users to set the mass-resolving power for the SIM measurement. Normally, improving the mass-resolving power enhances the ion selectivity and makes the ion in question less affected by interference from other ions having close mass-to-charge ratios. However, it also decreases the amount of ions to be reflected in the peak, which lowers the intensity of the ion and deteriorates the sensitivity.
Thus, a trade-off between the mass-resolving power and the sensitivity exists in the setting of the mass-resolving power. There is no general rule for determining whether the sensitivity should be to some extent sacrificed to improve the mass-resolving power and thereby enhance the quality of the extracted ion chromatogram (which is required for a better determination of the quantities of target components, as already explained), or conversely, the mass-resolving power should be to some extent sacrificed to improve the sensitivity. This is because it depends on the kind and state of the sample to be analyzed, the analysis conditions (e.g. the kind of mobile phase used in the chromatograph), and other factors. Until the sample is actually measured and data are collected, it is impossible to know what level of mass-resolving power is appropriate for obtaining a high-quality extracted ion chromatogram. Accordingly, to optimize the quality of quantitative determination, users need to perform the measurement a plurality of times and find an appropriate mass-resolving power.
This problem also occurs in the case of performing an MS/MS analysis in a multiple reaction monitoring (MRM) measurement mode or selected reaction monitoring (SRM) measurement mode using a triple quadrupole mass spectrometer as the detector for a chromatograph. In this case, users can set both the mass-resolving power of the first quadrupole mass filter and that of the second quadrupole mass filter. Accordingly, to optimize the quality of quantitative determination, they need to do an even more cumbersome task of performing the measurement a plurality of times while changing the combination of the mass-resolving powers of the first and second mass filters.